Wireless local area networks (WLANs) are expected to become the main method of conveying data in the near future. However, to provide continuity of service with current or future cellular systems such as GPRS or UMTS, one will need to couple WLAN and cellular systems.
One significant problem foreseen for dual WLAN/cellular terminals is power consumption. Among the various components within a typical mobile terminal that consume significant power is the analog to digital converter (ADC).
In a typical implementation of IEEE802.11a or BRAN HIPERLAN/2 WLAN, two ADCs are required for separate in-phase and quadrature-phase (I/Q) paths.
As an example, Analog Devices manufacture the AD775 ADC, whose specification states that it provides 20 Msamples/sec at 8-bit resolution, requiring 60 mW power—which is described in the specification as ‘very low’.
Within the WLAN frameworks mentioned, this equates to a total ADC power consumption of 120 mW.
This level of power consumption will reduce the lifetime of a battery within the terminal as compared with a similar terminal only connected to a cellular network. Possible solutions include using a larger battery, which is likely to result in a larger, heavier terminal, or find means to reduce the power consumption of the ADC within the dual WLAN/cellular terminal.
More generally, lower power consumption within ADCs and to a lesser extent digital to analog converters (DACs) is desirable in many portable devices, such as voice recorders, portable stereos and transceivers.
U.S. Pat. No. 4,395,732 discloses a system in which ADC quantization levels are adapted to a changing distribution of the analogue input signal so that each threshold is statistically as significant as every other threshold and therefore the quantizer's dynamic range is not wasted. In particular, the specification proposes to adapt the ADC quantization levels after each line of a television image. However, there is no description of how a single adaptation is performed; in particular, it is not described what algorithm is used to perform each single adaptation of the thresholds.
The present invention addresses the above problem of reducing ADC/DAC power consumption.